The infections due to methicillin-resistant Staphylococcos aureus (MRSA), previously limited to hospital areas and nursing homes, are now occurring in other public areas such as schools. Difficult- to-kill S. aureus strains are responsible for more annual deaths in the United States than those caused by HIV/AIDS. Therefore, novel therapies are urgently needed to meet the challenge of MRSA. Natural antimicrobial peptides ([1450] entries in our updated antimicrobial peptide database at http://aps.unmc.edu/AP/main.html) are an ancient and potent weapon against pathogenic bacteria. The protective role of the only human antimicrobial cathelicidin (LL-37) against infectious diseases has been firmly established in preclinical and clinical models. The long-term goal of our research is to elucidate the structure-activity relationship of human host defense peptide LL-37 as a basis for designing novel antimicrobial therapies. Our central hypothesis is that the 3D structure of LL-37 forms the basis for understanding its antimicrobial activity and provides a basis for peptide design. The middle region of LL-37 can be optimized and used as a template for engineering selective and potent antimicrobial peptides against MRSA by targeting and disrupting bacterial membranes. To test our hypothesis, we have designed the following Specific Aims: (1) To elucidate the relationship between three-dimensional structure and antimicrobial activity of human cathelicidin LL-37 and its fragments, (2) To engineer novel antimicrobial peptides against MRSA based on the LL-37 template, and (3) To investigate the mechanism of action of the engineered peptide and to test its in vivo efficacy in mice. These aims are framed based on preliminary results accumulated in the past several years in the PI's lab, as well as on the seminal contributions from the Co-I's lab. To achieve these aims, a strong team has been established to provide complementary expertise. Thus, the outcome of this novel research has a great potential in providing potent antimicrobial agents against S. aureus. Our research results will be published and deposited to the NIH so that they will be available to the public for a broad dissemination.